Lead for centuries has played an important role in making glass the common, easily formed, low cost material that it is. Lead in glass lowers the softening and melting points of glass making the glass workable at lower temperatures. There is now an environmental effort to eliminate lead from products commonly discarded in landfills, including glass products such as electric lamps. Conceptually, the elimination of lead from glass is simple. One needs merely to remove lead from the existing formulations and melt and work the lead free formulation at a higher temperature. Unfortunately, this is not a practical solution. For example, lead glasses are commonly used to make electric lamps, where the lamp cost depends largely on the raw material cost, the speed at which glass can be formed (equipment utilization speed) and the energy cost of processing the glass. Simply removing lead from glass formulas would require re-equiping factories to operate with higher temperature glasses and result in higher fuel costs for the manufacturer.
Most glass tubing used to make electric lamps is made by the Vello tube drawing process. Lead glasses such as SG10 and SG12 usually work well on the Vello process because they typically melt at lower temperatures and melt more efficiently than lead free glasses due to the fluxing ability of lead oxide. The Vello process produces tubes with excellent dimensional quality at very high drawing speeds, but forms the glass tubing at relatively high viscosities. Hence, glass compositions used on the Vello process must have low liquidus temperatures. In particular, the Vello process has been found to work best when the glass being formed has a viscosity of about 50,000 poise. It is also desirable to have a 50.degree. C. or more temperature difference between the liquidus temperature and the working temperature. Additionally, in order to manufacture the large variety of incandescent and fluorescent lamp products, glass tubing must be reworked on a variety of lamp working machines. This equipment must be versatile since there are thousands of lamp shapes which must be manufactured by the same process. Reworking the glass tubing to form flares, mounts, and blown bulbs requires a glass with a long working range, that is, one where there is a large temperature difference between the softening temperature and the working temperature of the glass.
Most known lead free glasses would be impractical to use in existing lamp making equipment since their melting and softening points are too high. In order to accommodate these more viscous materials, glass forming machinery would need to be modified and pressing, bending, and blowing operations would need re-adjustment. As a result, simple elimination of lead would drive up the manufacturing cost of electric lamps substantially. For example, most lead free and low lead glass compositions have short working ranges. Glasses with short working ranges tend to result in products with poor dimensional control and many glass defects. As such, a short working range requires very close temperature control at every production (index) station in the lamp manufacturing process. The labor and equipment required to monitor the lamp manufacturing process properly would raise manufacturing costs. For these reasons, there is a need for a lead free glass that has nearly the same mechanical working characteristics as the lead glass already in use.
In addition to the mechanical working properties of the glass, the glass must satisfy other criteria in order to produce high quality electric lamps. In particular, high electrical resistivity is required in the lamp glass which is used to make wire seals. The high resistivity prevents alkali migration during lamp operation. Alkali migration, also commonly referred to as electrolysis, can result in the glass cracking and lamp failure. High wattage incandescent lamps are the most prone to electrolysis since they typically operate at high temperatures and voltages. Although lamps might be redesigned to prevent electrolysis failures, the cost required to redesign all high wattage lamps and to make the necessary machine changes to manufacture the new lamps would be cost prohibitive. In addition, the glass must seal with the leads that penetrate the envelope to provide electric power to the light source. Large differences in thermal expansion can cause the glass to crack and break the hermetic lamp seal. Air then penetrates the envelope and oxidizes the filament. Consequently, there is a need for a lead free glass having a high electrical resistivity and a thermal expansion matched to that of the electric lamp leads.
Lastly, lamp manufacturers typically produce both low and high wattage incandescent lamps. As discussed above, high wattage lamps require a glass having a superior high temperature electrical resistivity. In the case of lead glass, SG12 is typically used. However, the high lead and potassia content of SG12 make this a more expensive glass type which restricts its use only to applications requiring the higher resistivity. As a result, manufacturers must use more than one type of lead glass to manufacture both low and high wattage lamp types, e.g., SG10 and SG12. This leads to increased inventory costs because of the need for more than one glass type and added manufacturing costs because of the need to adjust the sealing process for each glass. Thus, it would be advantage to have a lead free glass which could be used equally in the manufacture of low and high wattage electric lamps.
Examples of the prior art are shown in the following references.
U.S. Pat. No. 2,877,124 to Welsh, teaches lead free glasses that do not have the high electrical resistivities required for lamps.
U.S. Pat. No. 4,089,694 to Thomas et al. describes lamp glasses containing small amounts of baria and lithia as a replacement for lead in glass. The K.sub.2 O to Na.sub.2 O molar ratios are quite low, so the electrical resistivities are insufficient for most incandescent lighting applications. Fluorine was also required to improve glass melting and workability. Fluorine is undesirable since it is volatile and hazardous during glass melting.
U.S. Pat. No. 3,252,812, to DeLajarte, teaches lead free glasses that do no include lithia. The liquidus temperature of the DeLajarte glasses range from 1070.degree. C. to 1146.degree. C. which are too high, and the glass too soft for the Vello tube drawing process. The DeLajarte compositions would therefore devitrify prior to being Vello formed into tubing.
Kokai Pat. No. Sho 58 [1983]-60638 to Kawaguchi et al concerns lead free glasses suitable for manufacture of fluorescent lamp bulbs. They claim that their glass with BaO above 3.0 weight percent results in devitrification of the composition. The high electrical resistivities required for many incandescent lamps forces the BaO concentrations above this level.
Kokai Pat. No. Sho 57 [1982]-51150, to Sakamoto and Hayami also teaches lead free glasses for fluorescent lamp bulbs. With their compositions, BaO below 10.1 weight percent resulted in a more constricted, poorer working glass.
U.S. Pat. No. 5,391,523 to Marlor, which is incorporated herein by reference, discloses a lead free glass, SG64, which is an acceptable replacement for conventional lead glasses in most lamp types. However, the resistivity of the SG64 glass is insufficient for the mounts in high wattage incandescent lamps. Thus, it is still necessary for lamp manufacturers to use a high resistivity leaded glass, e.g., SG12, for such applications.
U.S. Pat. No. 5,470,805, to Filmer, describes lead free glasses, in particular one which is a commercially available lead free glass, PH360. Although the glasses cited can be used successfully to manufacture a wide variety of incandescent and fluorescent lamps, the electrical resistivity is too low for many higher wattage incandescent lamp types.